Subsea oil and gas wells are frequently drilled at an early time and riser pipes are added to extend them to surface facilities which are installed later. Production of the wells would commence up the riser pipes after the surface facilities are installed.
Primary reasons for the early drilling of subsea wells are exploration to attempt to locate oil or gas reservoirs and predrilling while waiting on the manufacture of the surface facilities. In the later case as soon as the investment in the surface facilities is completed, return cash flow can be quickly started.
The surface facilities of interest can be fixed steel platforms, fixed concrete platforms, tension leg platforms, floating production facilities, or other styles. Each of these extend the well from the ocean floor to the surface by use of riser pipes.
A problem has historically existed in the connection of the riser pipes to the subsea facilities. The problem is misalignment of the approaching riser pipe to the axis of the subsea facilities. Angles are frequently seen in the range of one to two degrees. This misalignment can be caused by misalignment of the facilities or due to current forces on the riser pipes.
Conventional threads such as the typical square threads used to run the wellhead housing portion of 18 3/4" systems provide 0.010" minimum clearance between adjacent threads when measured parallel to the centerline of the connections. On a 19.50" mean diameter thread, the arctangent of this clearance divided by the mean diameter of the thread (arctan(0.010/19.5)) yields an allowable angle of 0.02938 degrees. More reasonably stated this is 1/34th of a degree. This is 1/68th of the typically required alignment.
Before these threads can be screwed together in a 1 or 2 degree mismatch condition, the pipes must be bent into alignment. Guides on platforms have been used to attempt this straightening process. In some cases cylindrical extensions on the threaded connections themselves are used to attempt this alignment process.
Bending the typical 20" pipes in a 2 degree angle, shoving the male and female thread leads into contact under the bind, and then screwing them together is a difficult and risky process at best. A specific problem with this process is that as the cylindrical extension are shoved together from hundreds of feet above, the lead thread will likely be impacted with a high force. If the orientation of the threads is such that only a small portion of the lead thread is actually contacted, permanent damage can be imparted to both pieces. In most cases the subsea facilities half of the thread is not retrievable. Grinding the thread by divers in deep water for repair will be an expensive and time consuming operation.